In a valve operating mechanism of the internal combustion engine, a lash adjuster is employed for correcting change in the gap between valves caused by abrasion, thermal expansion and the like. However, in the lash adjuster, if oil containing air is fed into a high-pressure chamber disposed at a lower portion of the lash adjuster, the lash adjuster cannot fulfil its function. In order to prevent air from being entrained into oil, each of a patent publication No. 1 and a patent publication No. 2 propose a method comprising the steps of inserting a cylindrical sleeve into a plunger cap and separating air contained in oil from the oil entering the high-pressure chamber.